X-ray Variability of Mrk 421 in 2025 January: A Ten-Day XPoSat XSPECT Campaign with Contemporaneous NICER Coverage

We present an X-ray study of the high-synchrotron-peaked blazar Mrk 421 using ten days of XPoSat/XSPECT observations (2025 January 20-29; 202 ks) and contemporaneous NICER data. The 0.8-15 keV light curve has a fractional variability amplitude of 19.6 +/- 0.3 per cent, and Bayesian blocks resolve 26 flux states spanning a factor of 2.3. The best-constrained flare has a fast rise and a slow decay, with e-folding times of about 3 h and 30 h. For typical jet parameters, the X-ray-emitting electrons cool faster than this, suggesting that the decay traces a gradual weakening of their acceleration. The count-rate distribution is best described by two lognormal components, consistent with multiplicative variability. The power spectrum is a red-noise power law with a slope of about 1.7 over timescales of 25 min to 10 d, typical of blazars. XSPECT and NICER count rates are well correlated over eight matched epochs (r = 0.941). We fitted log-parabola models to day-resolved spectra from XSPECT, NICER and both jointly, and to flux-resolved XSPECT spectra. Both show spectral hardening with brightness. In the flux-resolved spectra, the photon index at 1 keV decreases from 2.24 in the faintest state to 1.96 in the brightest, and the synchrotron peak moves from about 0.66 to 1.06 keV, while the curvature ($β\approx 0.7$) shows no significant change. Power-law fits to Bayesian blocks show that bright blocks at the same rate can differ in photon index by up to $6.4σ$, so count rate alone does not fix the spectral state.